This invention relates to an optical recording medium applied to record and erase information by use of a recording layer changing its crystallographic state, and a method for preparing the same.
Recently, a variety of recording media have been proposed with high recording density and rewritable properties. One typical rewritable optical recording medium is of phase change type wherein laser light is directed to the recording layer to change its crystallographic phase whereby a change of reflectivity with such a crystallographic transition is detected. Optical recording media with the phase change materials have been gradually attractive because of not only the capability of a single beam overwriting, but also a relatively simple optical system as compared with magnetooptical recording system.
Ge-Te or Ge-Te-Sb have been widely used as the phase change materials, because of providing a wide reflectivity change between the crystalline and amorphous states and the materials show good durability. It has been recently proposed to use new compounds known as chalcopyrites. Chalcopyrite type compounds have been investigated as compound semiconductor materials for years and have been applied to solar batteries and so on. The chalcopyrite type compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 of the Periodic Table and the compounds have two stacked diamond structures. The basic characteristics are well described, for example, in "Physics", Vol. 8, No. 8 (1987), pp. 441 and Electrochemistry, Vol. 56, No. 4 (1988), pp. 228.
H. Iwasaki et al have mixed one of the typical chalcopyrite, Ag-In-Te.sub.2 with Sb or Bi and applied these compound alloys to optical memory recording at a liner velocity of 7 m/s. See Japanese Patent Application Kokai (JP-A) No. 240590/1991, 99884/1991, 82593/1991, and 73384/1991. More particularly, JP-A 240590/1991 discloses an information recording medium having a recording layer predominantly comprising (AgInTe.sub.2).sub.1-a M.sub.a wherein M is Sb and/or Bi and 0.30.ltoreq.a .ltoreq.0.92 and having a mixture of AgInTe.sub.2 and M phases. The alleged advantages include improvements in laser writing sensitivity, complete erasability, overwriting properties and the erasing ratio.
Optical recording disks which can be recorded and reproduced especially at the same linear velocity (1.2 to 1.4 m/s) of the present compact disk (CD) have been greatly interested because of sharing the same drive unit of the present CD by adjusting or modifying the optical system. write-once disks of this type have been developed by use of an organic dyes film as the recording material. However, rewritable one has not been proposed nor developed yet. The above-mentioned optical recording disks with the optical phase change have been expected well for this application because of its good erasability and unnecessity of a complicated drive system like magneto-optical system.
In Example of JP-A 240590/1991 referred to above, signals were recorded, while rotating the disk at a linear velocity of 7 m/s. However, when we tried to reduce the linear velocity to that used in CD with the same structured disk, C/N ratio dropped so rapidly and showed poor repetition of overwriting in comparison with the obtained properties at 7.0 m/s. This means that the film composition claimed in the above-mentioned application shows superb properties on recording and erasability at 7.0 m/s, however, the crystallization time of the film does not fit to the recording at a linear velocity of 1.4 m/s, because the crystallization time mostly depends on the alloy composition. Also the crystallization time relies on the disk structure. It is necessary to apply so called "a rapidly cooling strucutre", to solidify the amorphous state after the melting by the incident laser power.
This strucuture has already been known well, for example in Jpn. J. Appl. Phys. vol. 31(1992) pp 476-481. In this structure, a metal reflector with highly thermal conductivity is closely placed on a recording layer separated by a thinner dielectric film in order to cool down and solidify quickly to the amorphous state after the melting. A recorded mark is mostly solidified with a clear shape or contrast to the other area unrecorded, which is the crystalline state.